Relatively efficient photovoltaic (“PV”) cells can be manufactured in the laboratory; however, it has proven difficult to commercially scale these processes with the consistent repeatability and efficiency critical for commercial viability. The lack of an efficient thin-film manufacturing process has contributed to the failure of PV cells to effectively replace traditional energy sources in the market. Translating laboratory batch processing methods into effective industrial processes that are both cheaper and better controlled would help advance PV technology to mainstream markets.
Without an efficient thin-film manufacturing process, PV cells cannot effectively replace current energy sources. To manufacture a PV cell, a thin semiconductor layer of PV materials is deposited on a supporting layer such as glass, metal, or plastic foil. Since thin-film direct bandgap semiconductor materials have higher light absorptivity than indirect bandgap crystalline semiconductor materials, PV materials are deposited in extremely thin consecutive layers of atoms, molecules, or ions. The basic photovoltaic stack design exemplifies the typical structure of a PV cell. In that design, the cell comprises a substrate, a barrier layer, a back contact layer, a semiconductor layer, alkali materials, an n-type junction buffer layer, an intrinsic transparent oxide layer, and a conducting transparent oxide layer.
Compounds of copper indium diselenide (CIS) with gallium substituted for all or part of the indium (CIGS) and/or sulfur substituted for all or part of the selenium (CISS) have the most promise for use in absorber layers in thin-film solar cells. CIGS cells have demonstrated the highest efficiencies and good stability as compared to other absorber layer compounds. Typically, CIGS films are deposited by vacuum-based techniques. However, the multiple layers comprising a PV device offer challenges to a mass production system. Presently, there is no proven technology for continuously producing CIGS devices. Additionally, the typical PV cell manufacturing technique involves batch processing that necessitates touch labor, high capital costs, and low manufacturing output. In contrast, a continuous process can minimize capital costs and touch labor while maximizing product throughput and yields.
CIGS systems, in particular, pose unique challenges to manufacturers. As discussed by Ramanathan, et. al., Oct. 14, 2002, processes used for large area module manufacture involve deposition of metallic precursor stacks and the subsequent formation of the compound in a selenium and sulfur ambient. In photovoltaic applications, the p-type CIGS layer is combined with an n-type CdS layer to form a p-n heterojunction CdS/CIGS device. However, this process is problematic. The band gap of the CdS layer is still low enough to limit the short wavelength part of the solar spectrum that can reach the absorber, and this leads to a reduction in the current that can be collected. This reduction becomes proportionally more severe for higher band gap CIGS cells. Moreover, this process creates hazardous waste, the disposal of which is a challenge to potential manufacture. Thus, finding a practical alternative to the chemical bath deposition (“CBD”) CdS processes is desired in the art.
While some alternatives to the CBD CdS process have been proposed, none are viable options in the context of large scale continuous manufacturing. Some of these include the addition of layers composed of, among others, ZnS, ZnO, Zn(S,O), ZnSe, In2S3, and In(OH)xSy. However, inserting these alternative buffer layers often involve more chemical steps, as well as post deposition anneals or light soaking to become fully active. Adding these post-deposition steps decreases the efficiency of the manufacturing process, and subjects the product to potential mishandling and contamination. Thus, a process for manufacturing thin-film solar cells using an alternative to CBD CdS technology to insert a buffer layer—without additional chemical steps—is desired in the art.